A white LED (Light Emitting Diode) in which a blue LED using a nitride semiconductor light-emitting device and a phosphor having a yellow wavelength are combined together is a product with sales of 100 billion yen all over the world as a light source of a backlight applied to a liquid-crystal display of a cell phone. In addition, the white LED has characteristics of lower power consumption and longer operating life compared to a fluorescent lamp and an incandescent lamp. Accordingly, the white LED has been actively studied and developed as an alternative light source to a fluorescent lamp and an incandescent lamp.
As shown in FIG. 6, a nitride semiconductor light-emitting device is configured to include an n-type nitride semiconductor layer 220, an MQW active layer 230, and a p-type nitride semiconductor layer 240 sequentially on a growth substrate 210.
This nitride semiconductor light-emitting device is a type of a nitride semiconductor light-emitting device in which light is emitted in every direction from the MQW active layer 230. For example, the nitride semiconductor light-emitting device can also be used as a flip-chip type nitride semiconductor light-emitting device in which light is emitted from the MQW active layer 230 toward the p-type nitride semiconductor layer 240, when the nitride semiconductor light-emitting device is rotated 180 degrees upside down so that the p-type nitride semiconductor layer 240 can be located on the lower side.
As the growth substrate 210, a non-conductive substrate (for example, a sapphire substrate or the like) is used. Accordingly, the nitride semiconductor light-emitting device needs to secure a pathway for the electric currents to flow through the n-type nitride semiconductor layer 220, the MQW active layer 230, and the p-type nitride semiconductor layer 240 without involving the growth substrate 210 (hereinafter, the n-type nitride semiconductor layer 220, the MQW active layer 230, and the p-type nitride semiconductor layer 240 are referred to as a nitride semiconductor layer).
To be more specific, after the nitride semiconductor layer is formed on the growth substrate 210, an etching is performed on a part of the nitride semiconductor layer from a side of the p-type nitride semiconductor layer 240, until the n-type nitride semiconductor layer 220 is exposed. Then, an n-electrode 260 is formed on the exposed surface of the n-type nitride semiconductor layer 220. Thereby, the nitride semiconductor light-emitting device is formed. This nitride semiconductor light-emitting device can secure a conduction path without involving the growth substrate 210.
However, in a conventionally used nitride semiconductor light-emitting device, the n-electrode 260 and a p-electrode 250 are formed to be located at a pair of opposing corners on the nitride semiconductor layer side of the growth substrate 210. Since the electric currents have a characteristic of flowing a shorter path under the same resistance value, there has been a problem that the electric currents are concentrated in a portion corresponding to a line drawn from the p-electrode 250 to the n-electrode 260, thereby the electric cannot be distributed evenly.
This nitride semiconductor light-emitting device also has a problem that the electric currents are concentrated in a portion corresponding to a line drawn from the p-electrode 250 to the n-electrode 260, thereby emitting light evenly from the MQW active layer 230 is difficult.
Furthermore, this nitride semiconductor light-emitting device has another problem that the voltage is concentrated in a portion corresponding to a line drawn from the p-electrode 250 to the n-electrode 260, thereby an electrostatic breakdown is tend to occur in this portion.
In order to solve the aforementioned problems, discussions have been conducted on a nitride semiconductor light-emitting device having a conductive growth substrate, and further including: a p-electrode on one end of the nitride semiconductor layer, and an n-electrode on the other end opposite to the nitride semiconductor layer, with the growth substrate interposed therebetween. However, there is a problem that the manufacturing cost of such a nitride semiconductor light-emitting device becomes high, since SiC and the like serving as the conductive growth substrate are expensive.
As a manufacturing method for solving this problem, the following manufacturing method is disclosed (for example, refer to Japanese Patent Application Publication 2004-512688). In this method, after a nitride semiconductor layer is formed on a growth substrate, and a support substrate is joined onto the nitride semiconductor layer, an excimer laser light having a wavelength of approximately 300 nm or below is irradiated at several hundred mJ/cm2 onto the growth substrate side so as to thermally decompose the nitride semiconductor layer in the vicinity of the interface between the growth substrate and the nitride semiconductor layer. Thus, the nitride semiconductor layer is detached from the growth substrate 210.
By using such a manufacturing method, for example, a nitride semiconductor light-emitting device illustrated in FIG. 7 is prepared. This nitride semiconductor light-emitting device includes: a support substrate 370 on a p-type nitride semiconductor layer 340 side of a nitride semiconductor layer; and an n-electrode 360 on an n-type nitride semiconductor layer 320. The n-electrode 360 is connected to a wire, and supplies electrons to the n-type nitride semiconductor layer 320.
In this nitride semiconductor light-emitting device, the support substrate 370 functions as a p-electrode, and allows electron holes to flow across the entire surface of the p-type nitride semiconductor layer 340. Accordingly, uniformity of the electric currents to flow through the nitride semiconductor light-emitting device is improved.
Furthermore, since a reflecting mirror film 350 is provided between the support substrate 370 and the nitride semiconductor layer, the nitride semiconductor light-emitting device can emit a large amount of light from an MQW active layer 330 toward the n-type nitride semiconductor layer 320.
In the meantime, the nitride semiconductor light-emitting device includes the n-electrode 360 made of metal. This is because the optically-transparent electrode cannot be in ohmic contact with the n-type nitride semiconductor layer 320 if an optically-transparent electrode, such as ZnO and ITO, is used as the n-electrode 360 in the nitride semiconductor light-emitting device.